Thanks for the feedback, I think that's the way I'll go then.
Coming from a web background I think I'm too used to just injecting HTML into modal windows. I keep trying to figure out how in Unity to serialize all the content and push it into the window. I guess the best way of handling it is to just make several popup window prefabs, one for each usage and content type. That works fine with the tk2dUIItem parameter then.
Cheers!